ALMA Memo 493 Finding Fast Switching Calibrators for ALMA

In order to perform fast switching phase calibration effectively, we need to have a database of bright point-like sources at millimeter wavelengths. Optimally, this would include flux measurements at 86 GHz and spectral information at higher frequencies. We present a sample of 31145 compact flat spectrum radio sources with −40 < δ < +60, presumed extragalactic, which will be likely candidates for calibrator sources. A sample of variable steep spectrum source (ie, sources that could harbor a flat spectrum core even though they appear to be steep spectrum) adds another 1767 sources. And a sample of weak but highly inverted sources adds another 5296 potential sources to observe. Hence, we have as many as 38208 sources which are likely to be detected at millimeter wavelengths, though some of the steep spectrum variable sources and highly inverted source candidates may be abandoned based on detection rate. The Prototype Interferometer (PI) at the ALMA Test Facility (ATF) may be available to verify if these sources are indeed bright at 86 GHz. The sensitivity of the prototype receivers should permit a noise level of about 3.5 mJy at 86 GHz and about 10 mJy at 250 GHz. We expect about a third of the sources will be brighter than 20 mJy at 86 GHz, and these detections will be reobserved at 86, 106, 215, and 263 GHz to determine the spectrum at higher frequencies and to have some data on variability. These observations should take about 80-90 days of time on the Prototype Interferometer, and it is suggested that these observations could be done at night in the spring and fall of 2005, subject to availability of equipment and manpower.